Isomeriс Effects in the Mass Spectra of Stimulated Dissociation of Sulfonium Cations Desorbed from S-Alkylated Thiacyclanes under MALDI Conditions: A Convenient Method for the Derivatization of Sulfides with Alkyl Formates

Preliminary transformation into sulfonium salts by an electrophilic reaction with alkyl formates in the presence of trifluoromethanesulfonic acid is proposed for the study and analysis of non-polar organic sulfides by matrix-assisted laser desorption/ionization (MALDI). The test samples are thiabicyclanes with different positions of the sulfur atom and methyl substituents (2- and 3-thiabicyclo[4.4.0] decanes, 7- and 8-thiabicyclo [4.3.0] nonanes). The reaction used provide quantitative S-alkylation under mild conditions. In MALDI mass spectra of sulfonium salts, only peaks of corresponding sulfonium cations were observed, the masses of which corresponded to the sum of molecular weights of the sulfide and the increment of the introduced alkyl group. Drastic qualitative and quantitative differences were found and interpreted in the mass spectra recorded in the collision-induced dissociation mode for regio-isomeric sulfonium cations.